Recently, in order to take advantage of inexpensive CMOS, a system LSI in which a digital circuit and an analog circuit are combined on a single chip has been actively manufactured.
In such LSI, an A/D converter for converting an analog signal into a digital signal and a D/A converter for converting a digital signal into an analog signal are used at an interface between the LSI and the outside, with few exceptions.
Particularly in a LSI for video or communication, a current output type D/A converter capable of high-speed operation is indispensable. Generally, the current output type D/A converter has versatility in usage conditions.
To be specific, a resistor for output load and a resistor for current value setting are externally connected to the LSI so as to set analog output current and voltage according to actual usage conditions of the LSI, and further, a reference voltage for current value setting is inputted.
In order to screen an initial failure of the current output type D/A converter, a burn-in test is executed under the state where a wafer that is completed through a diffusion process is packaged (hereinafter referred to as “package burn-in”).
FIG. 6 is a circuit diagram illustrating a conventional current output type D/A converter 100. In FIG. 6, three bits of digital signals are converted into analog signals.
The conventional current output type D/A converter 100 comprises a reference voltage input terminal VREF, a reference resistor connection terminal IREF, digital input terminals IN1˜IN3, a bias circuit 101 as a voltage generation circuit, a decoder 102, current supply transistors IS1˜IS7, differential switches SW1˜SW7, and an analog output terminal OUT.
The reference voltage input terminal VREF is a terminal for applying a reference voltage from an external power supply 103 to the bias circuit 101. The reference resistor connection terminal IREF is a terminal for connecting the bias circuit 101 and an external resistor 104.
The bias circuit 101 is a voltage generation circuit for generating a bias voltage Vb, in accordance with the voltage supplied from the power supply 103 and inputted to the reference voltage input terminal VREF, and the external resistor 104 connected to the reference resistor connection terminal IREF.
The decoder 102 decodes the three bits of digital signals inputted to the digital input terminals IN1˜IN3 to output differential switch control signals D1˜D7.
Current supply transistors IS1˜IS7 output currents in accordance with the bias voltage Vb supplied from the bias circuit 101 and inputted to the gate terminals of the transistors IS1˜IS7, respectively.
The differential switches SW1˜SW7 are switches which are turned on and off on the basis of the differential switch control signals D1˜D7 outputted from the decoder 102, and the switches SW1˜SW7 output the currents outputted from the current supply transistors IS1˜IS7 to an analog output terminal OUT or a ground power supply VSS.
The analog output terminal OUT outputs an analog current according to the digital input signal.
When subjecting the D/A converter constituted as described above to package burn-in, an output load resistor 105 having a predetermined resistance value and performing current-to-voltage conversion must be connected to the analog output terminal OUT, after a predetermined output voltage is supplied from the external power supply 103 to the reference voltage input terminal VREF, and the external resistor 104 having a predetermined resistance value is connected to the reference resistor connection terminal IREF.
That is, during the package burn-in for the conventional D/A converter, it is necessary to perform, at least, connection of the external resistor for output load that is required for operation of the current output type D/A converter, connection of the external resistor for current value setting, and external application of the reference voltage for current value setting.
In such package burn-in, however, since the burn-in test is carried out after packaging the D/A converter, even parts to be screened as initial failures must be packaged as well, resulting in excessive cost or an expensive package.
In order to achieve cost reduction, recently, a wafer-level burn-in test has been executed. The wafer-level burn-in test is different from the package burn-in test in that the burn-in test is carried out on a wafer where resistors and wires are directly connected to pads on a semiconductor chip.
Another example of wafer-level burn-in is disclosed in Japanese Published Patent Application No.Hei.6-5677. As shown in FIG. 2 of this literature, passive elements such as resistors are formed around a semiconductor chip in a semiconductor wafer, instead of connecting external resistors or the like from the outside of the wafer, and the passive elements such as resistors are electrically connected to an input/output pad.
In the wafer-level burn-in, however, there are many cases where a space for connecting resistors and wires onto the wafer cannot be secured because of restrictions such as narrow spacing between pads on the semiconductor chip, and therefore, it may become impossible to carry out connection of the external resistor for output load, connection of the external resistor for current value setting, and external application of the reference voltage for current value setting, resulting in difficulty in executing the wafer-level burn-in test for the current output type D/A converter.
Further, when the passive elements such as resistors are formed around the semiconductor chip in the semiconductor wafer and electrically connected to the pads on the chip, there occurs a problem that a space for placing the passive elements cannot be secured, and a problem that a circuit of passive elements for wafer-level burn-in must be designed for each LSI when the pad interval or the like varies.